Just been trying to sign up to SS and they are asking for a scan of passport or similar ID (wish they'd ask at the beginning and not end of registration) - I've not been asked by any other sites for this.  What is this all about and is it legit?

This is fine and quite common. Some of the sites dont ask for the id at the start but do ask for it before you can get a payment.

i believe this helps them stop people from signing up as their own referral, signing up as a buyer and buying their own pics, and let's them know which country you are legally from for tax info.